We will be principally concerned with investigations of the protein neurophysin which serves as a carrier for the polypeptide hormones oxytocin and vasopressin within the hypothalamo-neurohypophyseal tract. These studies aim ultimately to interpret the non-covalent interactions between the hormones and the protein in terms of the thermodynamics of bonding of individual residues. Additionally they are concerned with elucidating the functional significance of the presence of duplicated segments within the neurophysin polypeptide chain. Present studies are most specifically concerned with identifying residues on the protein which interact with the hormones. Nuclear magnetic resonance spectroscopy, circular dichroism spectroscopy, affinity-labelling, and chemical and enzymatic modification are among the techniques to be employed for this purpose.